This invention relates to gas separation membranes comprising a copolyimide and the process for separating one or more gases from a gaseous mixture using such membranes.
Many industrial gas separation processes utilize selectively gas permeable membranes. Aromatic copolyimides have been suggested for use as the membrane material in some gas separations. Certain aromatic copolyimide membranes have been developed to provide high relative selectivities for one gas over another gas permeating through the membrane. Such membranes, however, suffer from having low gas permeation rates. On the other hand, different copolyimide gas separation membranes have much higher gas permeation rates, but they exhibit correspondingly lower relative gas selectivities.
In addition to good gas separation characteristics, commercially important processes often impose other formidable demands on the membrane material. For example, the purification of natural gas involves the separation of carbon dioxide from methane and/or nitrogen in the presence of liquid and gaseous hydrocarbons that contaminate the mixtures to be separated. The membrane material in this use should be highly resistant to the solvent effect of the hydrocarbon contaminants. Another important consideration is that the material should be easily fabricated into an appropriate membrane structure.
It is desirable to have a copolyimide gas permeation membrane which has both high selectivity and high gas permeation rates for gases being separated. It is also desirable that such a copolyimide membrane additionally has a strong resistance to hydrocarbon solvent activity. It is still further desired to have a copolyimide material that is readily fabricable to form membrane structures while preserving its combination of high selectivity, high gas flux and resistance to hydrocarbons
U.S. Pat. No. 4,690,873 discloses a gas separating material of copolyimide formed from recurring units of tetracarboxylic acids and moieties derived from diaminodimethyldiphenylene sulfone (xe2x80x9cAMPSxe2x80x9d) isomers, which include the compound o-tolidine sulfone (xe2x80x9cTSNxe2x80x9d). A glossary of selected chemical compounds referenced in this application is found in Table III, below. The compositions do not include 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (xe2x80x9c6FDAxe2x80x9d). The product membranes exhibit very high selectivity of carbon dioxide relative to methane but the carbon dioxide permeability is quite low.
U.S. Pat. No. 4,705,540 discloses polyimide gas separation membranes in which the membrane composition is polymerized from mixtures containing aromatic diamines and 4,4xe2x80x2-(hexafluoroisopropylidene)-bis(phthalic anhydride). This produces polyimides with extremely rigid chains. The monomer mixtures do not include blends of o-tolidine sulfone with other hydrophilic diamines. The membranes demonstrate very high carbon dioxide permeability but rarely carbon dioxide/methane selectivity above 25.
U.S. Pat. No. 5,042,992 discloses a gas separation material which is a polyimide formed by reacting 4,4xe2x80x2-(hexafluoroisopropylidene)-bis(phthalic anhydride) (xe2x80x9c6FDAxe2x80x9d) with diamino-dialkyldiphenylenesulfone. The material can form into membranes that have both high carbon dioxide permeability and high carbon dioxide/methane selectivity. The patent does not disclose a monomer mixture also containing a second, hydrophilic diamine, and further, it does not mention whether the polyimides are resistant to hydrocarbon solvents.
U.S. Pat. No. 5,591,250 discloses a process for separating carbon dioxide from a methane using a membrane of polyimide formed by reacting monomers of the single dianhydride 6FDA and one or more diamines. The use of o-tolidine sulfone as one of the diamines is not disclosed. Only rarely among the many examples are both high carbon dioxide permeability and high carbon dioxide/methane selectivity demonstrated. The resistance of the membranes to hydrocarbon solvents is not reported.
The present invention now provides a gas separation membrane of a copolyimide composition which advantageously provides a favorable combination of high selectivity and high transmembrane flux for commercially important gas mixtures while remaining resistant to attack from hydrocarbon chemicals. The copolyimide of the gas separation membrane is formed by copolymerization of monomers comprising o-tolidine sulfone, a hydrophilic diamine other than o-tolidine sulfone, and 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride. Optionally, one or more aromatic dianhydride can be included in the monomers utilized to form the membrane. The copolyimide is readily fabricable to a membrane form suitable for gas separation.
There is also provided a process for separating component gases of a gas mixture comprising the steps of
(a) providing a gas separation membrane of a polyimide formed by copolymerization of diamine monomers and dianhydride monomers in which the diamine monomers comprise o-tolidine sulfone and a hydrophilic diamine other than o-tolidine sulfone, and the dianhydride monomers comprise 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride,
(b) contacting the gas mixture on one side of the membrane to cause the component gases to selectively permeate the membrane, and
(c) removing from the opposite side of the membrane a permeate gas composition enriched in concentration of the component gases which are more preferentially permeable through the membrane.
The process of this invention is well suited to separating commercially important gases from gas mixtures and is especially valuable for separating carbon dioxide from methane and/or nitrogen in the purification of natural gas. The novel copolyimide gas separation membrane is advantageously resistant to hydrocarbon contaminants likely to be present in crude natural gas such as cycloalkanes, represented by cyclohexane, cycloheptane, cyclooctane, methylcyclohexane, methylcyclopentane and 1,2-dimethylcyclopentane, and aromatic hydrocarbons represented by benzene, toluene and xylene.
The present invention involves a gas separation membrane formed from a copolyimide composition. Generally stated, the copolyimide is produced by conventional process steps in which firstly a diamine and a dianhydride undergo polycondensation reaction to form a polyamic acid. Subsequently, the polyamic acid is dehydrated to obtain a polyimide. It has been discovered that production of the copolyimide from a particular selection of diamine and dianhydride monomers provides a gas separation membrane which exhibits superior gas separation, flux and hydrocarbon resistance properties.
At least two diamine monomers are utilized. One is 3,7-diamino-2,8-dimethyl diphenylsulfone, commonly known as xe2x80x9co-tolidine sulfonexe2x80x9d and, as mentioned, is sometimes referred to as xe2x80x9cTSNxe2x80x9d. At least one other diamine monomer is a hydrophilic diamine. The hydrophilic diamine can be aromatic, aliphatic or a combination of both. Preferably, the hydrophilic diamine has structure of formula I, as follows: 
in which R is an aromatic hydrocarbon radical of 6-24 carbon atoms, an aliphatic hydrocarbon radical of 3-12 carbon atoms or a mixture thereof, and X is a hydrophilic radical. By xe2x80x9chydrophilic radicalxe2x80x9d is meant that the pendant X group is highly polar. Great preference is given to hydrophilic diamine of formula I in which X is xe2x80x94OH, xe2x80x94SO3H, xe2x80x94CO2H, xe2x80x94NHR1, xe2x80x94NR2R3, or a mixture thereof, in which each of R1, R2, and R3 is an alkyl or aryl group. Combinations of such hydrophilic diamines are also contemplated and preferred. Representative hydrophilic diamines include 1,3-diamino-2-hydroxypropane (xe2x80x9cDAHPxe2x80x9d), 1,3-diaminobenzene-4-sulfonic acid (xe2x80x9cHSMPDxe2x80x9d), 2,2-bis(3-amino-4-hydroxyphenoxy)hexafluoropropane (xe2x80x9cbisAPAFxe2x80x9d), 3,3xe2x80x2-dihydroxybenzidine (xe2x80x9cHABxe2x80x9d), L-lysine, 1,3-diamino-5-benzoic acid (xe2x80x9cDABAxe2x80x9d), and mixtures thereof.
TSN is an ingredient in the dianine monomers utilized in all copolyimides of this invention. While not wishing to be bound by a particular theory, it is believed that TSN contributes to an optimized free volume in the polymer structure. This is thought to make the copolyimide highly gas permeable. Strong hydrocarbon solvent resistance and high permselectivity of the resulting copolyimide membrane are attributed to the presence of the hydrophilic diamine comonomer. It has been found that preferably about 20-80 mole %, and more preferably about 40-60 mole % of the total diamine monomers should be hydrophilic diamine and a complementary amount should be TSN.
In one aspect, the monomer mixture from which the copolyimide is derived includes the fluorine-containing aromatic dianhydride 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride, alternatively named 4,4xe2x80x2-(hexafluoroisopropylidene)-bis(phthalic anhydride) and, as mentioned, occasionally referred to as xe2x80x9c6FDAxe2x80x9d or xe2x80x9c6Fxe2x80x9d. In a preferred embodiment, 6FDA is the sole dianhydride reacted with the diamine components to form the copolyimide.
In another aspect, the dianhydride monomer can include one or more other aromatic dianhydrides in addition to and different from 6FDA. Preferably at least about 30 mole % and more preferably at least about 50 mole % of the dianhydride monomers should be 6FDA. Representative aromatic dianhydrides which are suitable for use in this invention include 1,2,4,5-benzene tetracarboxylic dianhydride (i.e., pyromellitic dianhydride or xe2x80x9cPMDAxe2x80x9d), 3,3xe2x80x2,4,4xe2x80x2-benzophenonetetracarboxylic dianhydride (xe2x80x9cBTDAxe2x80x9d), 3,3xe2x80x2,4,4xe2x80x2-biphenyl tetracarboxylic dianhydride (xe2x80x9cBPDAxe2x80x9d), diphenylsulfone dianhydride (xe2x80x9cDSDAxe2x80x9d), and bisphenol A dianhydride (xe2x80x9cBPADAxe2x80x9d).
The copolyimide can be made by methods well known in the art. In a preferred process, approximately equimolar amounts of dianhydride and diamine are reacted in a conventional polycondensation and dehydration polymerization scheme. Thermal imidization is preferred because it results in higher molecular weights of the polymer, allowing for easier membrane manufacture. The copolyimides of this invention have a weight average molecular weight of about 23,000 to about 400,000, and more preferably, about 50,000 to about 280,000.
Preferably, the diamines are first dissolved in a polymerization solvent medium and the dianhydride monomer or monomers is then gradually added portion wise under continuous agitation. The amount of solvent used should be sufficient that the concentration of the monomers is within the range of about 10 wt. % to about 30 wt. %, and preferably, about 20% by weight after combining all the monomers. Solvent can be added to achieve the desired concentration, if necessary.
The solvents which may be used in the polymerization process are organic solvents, and preferably anhydrous. The solvents should not react to any appreciable extent with the monomers, intermediates, product or other chemical species involved in the polymerization process. It is desirable that either the dianhydride or diamine monomer portions, and preferably both, are soluble in the solvent. Examples of suitable solvents are N,N-dimethylacetamide (xe2x80x9cDMACxe2x80x9d); N-methyl-2-pyrrolidone (xe2x80x9cNMPxe2x80x9d); gamma-butyrolactone; m-cresol, pyridine; diglyme; and like materials as well as mixtures of such solvents.
Polymerization is conducted under anhydrous conditions while agitating the mixture and maintaining the reaction mass at a temperature below about 50xc2x0 C., and preferably, in a range of about 20-35xc2x0 C. The reaction vessel can be immersed in a cooling bath to control the temperature. Polymerization is conducted for a time sufficient to form a polyamic acid having the desired molecular weight. This occurs usually within about 2 to about 20 hours. The polyamic acid may then be thermally converted to the polyimide by heating the polyamic acid solution to about 150-200xc2x0 C. until imidization is substantially complete. The polyimide may then be recovered from solution by precipitation with alcohol (e.g., methanol) or water and washed with additional alcohol or water.
It is helpful for forming the polymer into a gas separation membrane utilizing conventional techniques that the copolyimide is readily soluble in certain solvents. The preferred solvents are polar aprotic solvents. Representative examples include NMP and DMAC.
To prepare membranes according to this invention, the polymer is dissolved in an appropriate solvent and the solution can then, for example, be cast as a sheet onto a support, or spun through a spinneret to yield a hollow fiber. The solvent is then removed. The technique by which the solvent is removed affects the characteristics of the resulting membrane. For example, evaporating the solvent by heating produces a uniformly dense membrane. By comparison, quenching the film or fiber membrane precursor structure in a liquid which is a nonsolvent for the polymer and miscible with the solvent of the polymer solution will form an asymmetric membrane, i.e., a membrane in which the density varies with distance normal to the membrane surface.
Certain copolyimides according to this invention are not adequately soluble in solvents preferred for industrial processing, such as liquids which are nontoxic, noncorrosive, inexpensive, have proper volatility characteristics and are amply available. For those copolyimides, the novel copolyimide membrane may still be produced by dissolving the polyamic acid precursor in an appropriate solvent, forming the membrane structure as previously mentioned, and then, converting the polyamic acid to copolyimide, for example by heating the membrane structure.
The copolyimide membrane of this invention can be formed into a number of shapes well known in the art. Flat films and hollow fibers are preferred. Flat films can be self supporting within a frame or supported by a substrate which is usually porous. The flat film can be used in flat configuration. Other possible configurations for flat films include winding the film in a spiral form or pleating the film to generate a higher transmembrane surface area per unit volume. Hollow fibers can be bundled in parallel flow arrangement and potted in a tube sheet at each end. The tube sheet is inserted in a typically cylindrical case to form a hollow fiber gas separation membrane module as is well known in the art.
The novel membrane can be used to great advantage for separating gaseous components of gas mixtures. In a general sense this is accomplished by bringing the gas mixture in contact with one side of the membrane and allowing the components of the mixture to permeate through the membrane. Components of the mixture which are more preferentially permeable than other components will pass through more rapidly to form a so-called xe2x80x9cpermeatexe2x80x9d composition on the opposite side of the membrane. The permeate composition will thus be enriched in the faster permeating components. The gas composition on the first side, occasionally referred to as the xe2x80x9cretentatexe2x80x9d side, will become depleted in the faster permeating components. Frequently, a pressure gradient is imposed across the membrane from high pressure on the retentate side to low pressure on the permeate side. This increases the rate of permeation. The permeate and retentate compositions are withdrawn from the vicinity of the membrane for further processing, storage, use or disposal as called for any particular practical application. Considerations for operating gas separation membrane units is well understood by those of ordinary skill in the art.